1. Field of the Invention
The present invention relates to the preparation of granular fertilizer solids, and, more especially, to the preparation of granular NP/NPK fertilizers comprising ammonium phosphate.
2. Description of the Prior Art
It has long been known to this art to granulate fertilizers from liquid raw materials by utilizing any one of a number of acceptable techniques. One apparently simple method comprises effecting granulation and drying in a single unitary operation, as described, for example, in French Pat. Nos. 1,100,817 and 1,351,668 (U.S. Pat. No. 3,227,789). However, this type of procedure suffers from certain disadvantages, such as: a high water content in the slurry, limited production capacity, the impossibility of producing particular types of fertilizer, or certain formulae, and the impossibility of injecting a makeup amount of ammonia into the wet solid phase.
Consequently, efforts have been directed towards granulation processes performed in two separate operations, e.g., in a mixer (or granulator) and then a drier which are disposed in series in the same granulation circuit.
Unfortunately, when a large proportion of raw materials in liquid form (molten mass, aqueous suspension or solution of fertilizer species) is introduced into the granulator of a conventional granulating installation, a substantial amount of the dried material has to be recycled to the granulator in order to reduce the moisture content and the temperature of the material contained in the granulator, to a level which is conpatible with effective granulation. The recycling required typically ranges from 5 to 12 times the actual production.
French Pat. No. 1,389,361 (U.S. Pat. No. 3,348,938) proposes overcoming this disadvantage by spraying into the drier a portion of the liquid fertilizer species, in particular a slurry of monoammonium and diammonium phosphates. The required recycling of dried material is thus reduced to less than 5 times the production.
However, while this process is excellent in principle, it requires additional apparatus for neutralizing phosphoric acid with ammonia, comprising, for example, one or two stirred vessels. And water vapor produced by the heat of reaction has to be separated from the slurry and discharged by suitable means. It is also necessary to provide a pump for passing the slurry under pressure to the drier, a slurry flow meter, and spraying means for introducing the slurry into the drier.
The aforesaid slurry must be sufficiently fluid for it to be pumped and sprayed, and this requires a water content which is between 12 and 20% depending on the N:P atomic ratio in the mixture of monoammonium and diammonium phosphates.
When the aforenoted atomic ratio is 1.6, as set forth in said French Pat. No. 1,389,361, it too is known that loss of ammonia in the neutralization and drying steps is high, on the order of from 10 to 20% of the ammonia introduced. Such losses can be recovered in a suitable gas washing system, by means of phosphoric acid for example, but, in order for the liquid circulating in the washing system to remain sufficiently fluid, a substantial amount of water, which is proportional to the amount of ammonia to be recovered, has to be introduced. And this necessitates increasing the dimensions of the drier and enhancing heat consumption.
In the preparation of fertilizers, it is also known to use a tubular reactor in order to provide for contact between sulfuric or phosphoric acid and ammonia, and to separate the water vapor created from the slurry produced in a cyclone, as disclosed in U.S. Pat. No. 2,755,176, or in U.S. Pat. No. 3,310,371. Unfortunately, the cyclone separator is subject to blockage which makes the operation difficult.
It has also been proposed that ammonium sulfate and phosphate base fertilizers may be produced by means of a tubular reaction vessel for neutralization of the acids, communicating directly into the granulator without first separating off the water vapor, as described in U.S. Pat. No. 3,954,942. This process is likely suitable when using salts which are relatively weakly soluble, such as ammonium sulfate and phosphate; nonetheless, in order to avoid an excessive rate of recycling, a portion of the ammonium sulfate often has to be introduced in solid form, or a portion of the P.sub.2 O.sub.5 often has to be introduced in the form of solid superphosphate, as is apparent from the actual working examples of the aforesaid '942 patent. When the fertilizer incorporates substances which are highly soluble in a hot condition, such as ammonium nitrate or urea, which are introduced into the granulator in the form of a solution, direct projection of the water vapor generated by the heat of acid neutralization onto the bed of product in the granulator results in a considerable rise in the temperature and the humidity of the product in the granulator, and makes it necessary to maintain a high rate of recycling which is generally between 6 and 10 times the production, depending on the formulation being produced, and the water content of the raw materials used.
See also, generally, and for background purposes, U.S. Pat. Nos. 2,926,079, 3,092,489, 3,241,946 and 3,333,297; French Pat. Nos. 1,206,786, 1,128,993, 1,573,849 and 2,437,386; British Pat. Nos. 1,252,651 and 1,379,761; and Canadian Pat. No. 592,240.